can't adjust contrast in scanned photo

I've scanned some black and white photos on a Ricoh photocopier. It scanned them as .tiff. The files were lighter than the originals, with less contrast. GIMP told me Brightness and Contrast controls didn't work with indexed files. I converted them to .jpg.

The brightness control seems to work in both directions, but the contrast only works downwards. Oddly, if I adjust brightness downward and then contrast upward the contrast control doesn't increase contrast, but instead undoes the work of the brightness control and makes them image lighter again.